Another battery will extend your time to the same discharge level
Of both batteries
While then requires a longer generator runtime to recharge
Or
have the batteries at a higher level, for the same amount of use
If your not getting enough solar, to recharge one battery, you still need the generator to keep both batteries charged
Another battery will not change the amount of solar you get oto replace the amount you use
More battery capacity will make it better/easier on the batteries and they will have a longer life for the same amount of use
For more charging your need more solar and generator